Scientists at the University of Wisconsin are searching for evidence of alien life not by looking into outer space, but by studying rocks right here on Earth.

Researchers are looking for crucial information within the rocks - some of which are up to 3.5 billion years old - to understand how life might have arisen elsewhere in the universe and guide the search for life on Mars one day.

Geoscientist Clark Johnson, the lead investigator for the Wisconsin Astrobiology Research Consortium said: "There's a story always hidden in rocks. ... It's up to (geologists) to be clever enough to find the tools that we need to interrogate those rocks to find what story they preserve."

The project is funded through NASA, which provided a $7m (£4.5m), five-year grant that started in January. It was the group's second five-year, $7m grant.

The consortium's goal is finding footprints of biological activity, or biosignatures, which are substances such as elements or isotopes that show evidence of ancient life.

The scientists are looking for microscopic signs of life, including microbes, which are bacteria, and other tiny, one-celled organisms that are much more adaptable than more complex organisms.

The team is also sending microbes into Earth's orbit on the International Space Station to see how they react to radiation and a space environment.

In the process, they are learning more about Earth's history. They have found new details of microbial life that dates back two billion to three billion years, before the planet's atmosphere contained oxygen. They found that microbes then relied more on iron than sunlight for energy.

Eventually their work will be used to interpret data beamed back from Mars by the six-wheel spacecraft Curiosity, which landed in August on a two-year mission to determine whether the environment was ever favourable for microbial life.

Their work will also be used to prepare for future Mars missions.

Mr Johnson said: "It may be that planets spent a long time in a microbial life condition and then only rarely evolved to advanced multicellular complex life. That's one of the hypothesis we would test."

Edward Goolish, acting director at the NASA Astrobiology Institute, said the project supports one of NASA's major goals to find life or the potential for life elsewhere.

The project's results will provide a quantitative understanding of how life is preserved, he said.

"At the same time (Johnson's team is) contributing an immense amount to the understanding of life on Earth, which is equally important to astrobiology and science in general," he said.